Infectious diseases of the gastrointestinal tract are a significant cause of infant mortality and morbidity in the developing world. Globally over 1 billio people are infected with gastrointestinal helminths and diarrheal disease is the second leading cause of death in children under 5, killing approximately 2 million children every year. Vaccination is one of the most cost-effective health interventions that can protect people from infection in endemic areas, saving millions of lives every year. Although vaccines currently exist to protect against infection with rotavirus and cholera, uptake of these vaccines in developing countries is poor due to the financial cost. Furthermore there is an urgent need for the development of new vaccines that provide protection against other diseases of the gastrointestinal tract. This proposal will develop a new affordable vaccine delivery platform that uses the probiotic yeast Saccharomyce cerevisiae boulardii to deliver vaccines directly to the gastrointestinal tract. Using genetic techniques already established for Saccharomyces cerevisiae I will genetically engineer S. boulardii to express antigens that induce protective immune responses fused to a novel adjuvant technology, ImmunobodiesTM. Oral administration of genetically transformed S. boulardii will facilitate expression of these fusion proteins in situat the mucosal surface. The generation of immune responses by antigen-ImmunobodyTM fusion proteins delivered by genetically transformed S. boulardii via oral vaccination will be initially demonstrated in mice using model antigen to describe the baseline immune responses generated by different dosing regimens. Co-administration of S. boulardii expressing mouse polarizing cytokines will facilitate the arming of different the immune effector mechanisms driven by Th1, Th2 or Th17 responses. This will be important because different types of pathogens require different immune effector mechanisms for clearance. I will also examine the longevity of the immune responses generated by this vaccine delivery platform by assaying for memory responses (plasmablasts and central memory T cells) in mice responding to antigen challenge. The protective nature of the immune responses generated will be demonstrated using a mouse model of diarrheal infection, Cryptosporidium parvum. Defined vaccine targets against C. parvum will be expressed by S. boulardii as ImmunobodyTM fusion proteins and the vaccinated mice challenged with Saccharomyce cerevisiae boulardii infection. Lastly I will generate auxotrophic mutants of S. boulardii to facilitate large-scale production of this vaccine delivery platform in developing countries removing the requirement for costly antibiotic selection. This project will radically change the way vaccines are administered both in developed and developing countries.